For example, in order to destruct a surface film of steel materials represented by stainless steels coated with a remarkably strong oxidized film, whereby removing scale ingredients, it has been proposed to immerse them in a high-temperature composite salt bath. As for a method of treating a solution generated in this immersing treatment, a method, in which nitrate radicals causing a supernutrition and strongly poisonous oxidizing chromium salts are discharged in high concentrations and a system is maintained under the strongly acidic condition to separate these chromium salts followed by adding a reagent having a strong reducing power to turn said system basic again, whereby separating metallic ingredients, has been adopted. However, expensive reagents contained have not been recovered and great expenses have been spent in a removal of said metallic ingredients.
Of salt radicals contained in a discharged water, in the case where a method of acting them upon outside compounds to turn them into insoluble substances, whereby separating them out of the system, has been known, no problem occurs but in general it is remarkably difficult to find a method of insolubilizing them.
Accordingly, if said salt radicals can be easily separated from a discharged system or a separation in the form of solution is easy and an accuracy of separation is high, there is the possibility that it is received in remarkably wide fields also in respect of environmental problems. However, no practical art has been discovered.
According to the known art, in order to insolubilize dissolved metallic ions under the condition that a considerable concentration of soluble metal salt coexists in the presence of high concentration of free alkali, at first a large quantity of acid has been added to neutralize alkaline ingredients and additionally a system has been acidified followed by adding a strong reducing reagent on the market to reduce an ionic valence, whereby regulating the pH which has been carried out as the general method of insolubilizing said metal salts.
It has been disclosed in Japanese Patent Application Laid-Open No. Hei 2-145786 that free acids in a pickling bath are recovered by the use of a diffusion dialysis membrane while a reducing power of ferrous ion ingredients remaining in a solution of metal salt ingredients, from which generated free fractions have been removed, is utilized.
However, a method of effectively removing soluble metal salts containing a large quantity of nitrate radicals discharged from a salt-treating process has not been disclosed in Japanese Patent Application Laid-Open No. Hei 2-145786.
A high-temperature salt bath is obtained by melting solid substances at high temperatures. A surface treatment of a steel material is carried out by immersing said steel material in this salt bath and process products are carried out together with the steel material. Said products are washed with a large quantity of water to be dissolved in washings, whereby being removed.
As to a composition of this salt bath, sodium hydroxide and sodium nitrate are used in a ratio of 6 to 7:4 to 3 in equivalent. That is to say, sodium hydroxide is dominant. In addition, there is a tendency that a quantity of sodium hydroxide in equivalent is larger than the total quantity of acid radicals in equivalent used in the process, that is a discharged water from a factory is considerably alkaline. Accordingly, there has been a general tendency that acids must be purchased from outside in order to neutralize said discharged water from a factory.
An overflown water, which has been subjected to this washing treatment, contains not only dissolved expensive reagents but also chromium ions and manganese ions oxidized to be dissolved from the treated steel material in a high concentration even within a strong alkaline range. In order to remove these ingredients together with insoluble ingredients, at first a large quantity of coexisting alkaline ingredient is neutralized with acids on the market and then strong reducing reagents on the market are additionally added with maintaining an acidity. Thus, an operation requiring great expenses, in which a reducing atmosphere reducing ionic valences of metals is generated and metallic ions are turned to be alkaline again to insolubilize general metallic ions and then the insoluble general metallic ions must be separated from a solution, must be adopted.
This operation is sufficiently analyzed with the following problems:
A) A problem occurs in that a large quantity of expensive sodium hydroxide to be turned into valueless salts having a reduced utility value. An effective recoverying method should be proposed.
B) Not only coexisting nitrates are difficult to be singly separated when mixed in the discharging system but also they contain a large quantity of nitrogen radical causing a supernutrition becoming an environmental problem. Accordingly, their quantity discharged is regulated in some districts even though they are not treated as poisonous substances. However, no effective removal effect has not been expected even though great expenses are spent.
However, nitrate radicals are expensive salts but it is expected that they can be reused as nitric acid used in the process if the acid radicals can be separated. If an ion electrolytic dissociating operation is adopted in this operation, it is expected that not only the metallic ingredients accumulated in the acidic bath can be removed but also sodium ions, which have acted upon the nitrate radicals, can be removed.
C) In addition, this liquid contains a large quantity of poisonous oxidized salt of chromium and manganese and their removal is strictly regulated in view of the pollution control and thus it is required to completely remove it. It is said acids purchased from outside for neutralizing said coexisting alkalies that spend the greatest expenses in the insolubilizing operation after the reduction. If these expenses are reduced, not only the removal of the free alkaline ingredients described in said item A) but also a reduction of expenses can be achieved.
The next problem consists in that said reducing operation of the metallic ions turned to maintain their dissolved condition even in an alkaline atmosphere is simplified. This reducing operation is achieved by finding out an atmosphere depriving electrons from the side of the other party when ferrous ions are oxidized to be turned into ferric ions and carrying out a treatment for producing that condition to reduce said ionic valences of the soluble salts in an acidic atmosphere in which said ferrous ions coexists.
The atmosphere meeting this condition is set up depending upon conditions within a pickling tank arranged in succession to the producing line of the same one steel material. These conditions consist in that free acid radicals are contained in a quantity of 0.7 to 1.0 equivalent, a strong acidity being always maintained, a liquid having a temperature of 40.degree. to 60.degree. which is required for a reaction, and a large quantity of ferrous ions required as a reducing agent being contained. A pickling bath has a composition preferably containing suitable quantities of ferrous ions and ferric ions and this composition is effective for enhancing a pickling effect of the steel material to be treated. If such the environment can be utilized well, it is not required to purchase the expensive reducing agent from outside as in the prior art and it is not required to increase a quantity of sludge for the reducing treatment.
In addition, it has been found that the reducing operation from hexavalent chromium ions to trivalent chromium ions with the ferrous ions is carried out in the same one bath as in the descaling of the stainless steel material, so that scales remaining on a surface of the stainless steel material are removed and at the same time reduced chromium is stuck to an activated metallic surface to change scientific characteristics of said metallic surface.
On the other hand, reagents composing the bath carried out from the salt bath for the salt-treatment of the stainless steel are expensive and can not be turned into insoluble salts even by the neutralizing treatment and thus they become nitrate radicals as a nitrogen source causing a supernutrition when contained in the discharged water, so that their removal has been called in question.
Sodium nitrate mainly contained in these baths are soluble. Accordingly, in order to take out it from the system, a concentrating operation cannot but being used. In addition, even though it is concentrated, it can not be reused unless it is dehydrated in respect of its real application.
Furthermore, said reagents carried out from this salt bath include chromium molecules, which are one of the ingredients composing the stainless steel material to be treated, but these chromium molecules are turned from Cr.sup.3+ into Cr.sup.6+ because a treating temperature is high. This Cr.sup.6+ is soluble to water and strongly poisonous. Accordingly, its sure disposition as the pollution control is required but its control is difficult and thus a problem in view of disposition of wastes occurs.
Consequently, according to the prior art, Cr.sup.6+ is reduced to Cr.sup.3+ in the presence of the reducing agent and the resulting Cr.sup.3+ is removed by the cohering treatment. However, nitrate radicals have not been removed from the water system.
It has been proposed also that a ferrous salt on the market is used for the aimed treatment as the reducing agent of hexavalent chromium.
It has been disclosed in Japanese Patent Application Laid-Open No. Hei 2-145786 that free acids in a picking bath are recovered by the use of a diffusion dialysis membrane while a discharged liquid mainly containing metallic ingredients generated is utilized as a reducing-neutralizing agent. It has never been, however, disclosed in Japanese Patent Application Laid-Open No. Hei 2-145786 that nitrate radicals contained in the liquid, which has been subjected to the chilling treatment, are concentrated to separatedly recoverfree nitric acid and sodium hydroxide, whereby reutilizing them to reduce a quantity of reagents contained in the discharged water.
The reagents used in the salt bath are not only expensive but also has the characteristics that it is difficult to separate them from the solution followed by concentrating.
On the other hand, although an aged nitric acid family pickling liquid has been in general singly neutralized to be discharged, a part of iron contained in it effectively acts for the reduction of hexavalent chromium ions. Accordingly, said quantity of said reagents to be used can be reduced by mixing these liquids.